Exchange 2013 should be supported by AVG 2012.
But please note, there is an issue with SP1 for Exchange 2013:
In Exchange 2013 SP1 there is bug which is described here:
Fix must be done from microsoft side, it should be implemented in Exchange Server 2013 Cumulative Update 5. Confirm if you have the update, if you are missing the update, please read the following KB article: